In a wireless communication system, especially in a mobile communication system, fading occurs from times to times. Buildings, mountains, and foliage on a transmission path can cause reflection, diffraction, and scattering on a propagating electromagnetic wave. The electromagnetic waves reflected from various large objects, travel along different paths of varying lengths. If an obstacle on the transmission path has sharp irregularities, the secondary waves resulting from the obstructing surface are present around the obstacle. Also if there are small objects, rough surfaces, and other irregularities on the transmission path, scattered waves are created. All these waves interact with each other and cause multipath fading at specific locations. The multipath fading can seriously deteriorate the quality of a communication system.
Usually there are two methods to deal with the problem caused by multipath fading. One method is to use a multipath combiner such as a RAKE receiver to combine the significant paths of a multipath-fading signal together. Another method is to use an equalizer to eliminate all paths of a multipath-fading signal except the strongest path.
A multipath combiner in a receiver is to combine significant paths together. It works effectively when the most significant paths have almost same strength and these paths do not nullify. Looking at these paths on time domain, one can see that a transmitted symbol appears several times with each time corresponding to a different path. When transmission rate is low, the multipath fading spans in less than one symbol period. Through the multipath components interfere with each other, the transmitted symbols do not cancel each other totally. However, as the transmission rate is higher and higher, a multipath fading could easily span one or more symbol periods and a symbol on one path could be almost cancelled by a previous symbol on other path. When symbols almost cancel each other or possess nulls, a multipath combiner has nothing to combine and therefore system performance declines.
A decision-feedback equalizer is effective to compensate the nulls. Basically, an equalizer in a receiver is to keep the strongest path and eliminate all other paths. It works effectively when there is a strong and stable path.
However, when a multipath fading spans one or more symbol periods and no any path is stable, either multipath combiner or equalizer does not work effectively. One example for this kind of scenario is a high-speed mobile receiver in metropolitan area. In order to have reliable communication under this kind of scenario, one may want to make the adjacent transmitted symbols different and utilize a multipath combiner to combine all these significant paths together.
A direct sequence spreading spectrum system is able to make adjacent symbols different. The delayed versions of the transmitted pseudo-noise (PN) signal have poor correlation with the original PN signal and therefore a direct sequence spread spectrum system is multipath resistant. A multipath fading causes PN chips interfered with each other. The interference caused by one chip to other chips is called interchip interference. For detection is based on a group of PN chips, or on a symbol, the mess caused by multipath fading is less severe. Usually chip rate is much higher than symbol rate and a P N period is much longer than a symbol period, therefore it is possible to distinguish the components of a multipath fading signal spanned over several symbol periods. Together with a multipath combiner, a direct sequence spread spectrum system can combine the information obtained from several resolvable multipath components and therefore improve the system performance.
One disadvantage of a direct sequence spreading spectrum system is low spectral efficiency. In order to transmit data with higher rate, a multi-channel direct sequence spreading spectrum communication system is introduced.
In a multi-channel communication system, the multipath fading could be more serious. First, each component of a multipath-fading signal on a particular channel not only interferes with the other components of the signal on the same channel, but also interferes with each component of the signals on other channels. The interference caused by a component on a channel to other components on the same channel is called intersymbol interference. The interference caused by a component on a channel to the components on other channel is called interchannel interference. Second, even if each symbol on each channel has poor correlation with their neighbor symbols, a delay version of the current symbol on a particular channel could almost cancel some future symbol on one of the other channels and possess null on that channel.
Based on above discussion, it is interesting to employ both multipath combiner and decision-feedback equalizer in a multi-channel communication system.
One way to employ both multipath combiner and equalizer in a receiver of multichannel communication system is to use an equalizer and a multipath combiner for each channel. There are two problems. One problem is huge hardware consumption. For a multi-channel system with 64 channels, there will be 64 multipath combiners and 64 equalizers. Another problem is that each equalizer works independently, which may not be a good choice to subdue interchannel interference. In order to reduce interchannel interference, one may introduce some connections among these equalizers, which makes the design even further complex.
Another way to employ both multipath combiner and equalizer in a receiver of multi-channel communication system is to use an equalizer for selecting each significant path of a multipath fading signal and to use a multipath combiner to combine the signals from all of these equalizers together. There are some problems also. The several significant paths do not have equal power. The equalizer, which selects a path with relatively small signal strength, may not work effectively especially when the path is not stable. Also, in metropolitan area, usually at least 4 paths have to be considered and therefore there must be at least 4 equalizers. The hardware consumption is still a lot.
Therefore, in order to save hardware, it is attractive to use one multipath combiner to combine the signals of several significant paths together and use one decision-feedback equalizer to take care of interference in a multi-channel communication system.